try uploading all the files again. maybe one or more got corrupted or missed during your upload.
only use a real ftp program, not your host file manager.
unzip the phpbb file on your computer before you upload the files. do NOT unzip on the server.

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Warning: include(./../language/en/common.php): failed to open stream: No such file or directory in /home/u870464295/public_html/install/index.php on line 225
Warning: include(): Failed opening './../language/en/common.php' for inclusion (include_path='.:/usr/lib/php') in /home/u870464295/public_html/install/index.php on line 225
Warning: include(./../language/en/install.php): failed to open stream: No such file or directory in /home/u870464295/public_html/install/index.php on line 225
Warning: include(./../language/en/install.php): failed to open stream: No such file or directory in /home/u870464295/public_html/install/index.php on line 225
Warning: include(): Failed opening './../language/en/install.php' for inclusion (include_path='.:/usr/lib/php') in /home/u870464295/public_html/install/index.php on line 225
Warning: include(./../language/en/posting.php): failed to open stream: No such file or directory in /home/u870464295/public_html/install/index.php on line 225
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Warning: include(): Failed opening './../language/en/posting.php' for inclusion (include_path='.:/usr/lib/php') in /home/u870464295/public_html/install/index.php on line 225
Fatal error: Uncaught exception 'ReflectionException' with message 'Class phpbb\hook\finder does not exist' in /home/u870464295/public_html/vendor/symfony/dependency-injection/Symfony/Component/DependencyInjection/ContainerBuilder.php:957 Stack trace: #0 /home/u870464295/public_html/vendor/symfony/dependency-injection/Symfony/Component/DependencyInjection/ContainerBuilder.php(957): ReflectionClass->__construct('phpbb\hook\find...') #1 /home/u870464295/public_html/vendor/symfony/dependency-injection/Symfony/Component/DependencyInjection/ContainerBuilder.php(490): Symfony\Component\DependencyInjection\ContainerBuilder->createService(Object(Symfony\Component\DependencyInjection\Definition), 'hook_finder') #2 /home/u870464295/public_html/install/index.php(252): Symfony\Component\DependencyInjection\ContainerBuilder->get('hook_finder') #3 {main} thrown in /home/u870464295/public_html/vendor/symfony/dependency-injection/Symfony/Component/DependencyInjection/ContainerBuilder.php on line 957